CD-ROMs are manufactured for use in applications that require a ROM of very large capacity but do not require the modification of the recorded information such as the recording and playing of music and as well as the storage of encyclopedic computer data. Information may be stored as analog encoded data (typically FM) or as binary encoded data. Binary encoding is the preferred method for audio and computer data but they generally differ in the recording format and associated error correction technologies.
A typical CD is capable of storing up to 600 million bytes of information, an order of magnitude greater than a typical computer disk drive using a 120 mm (4.72 inch) diameter disc. Because of the extremely high density of tracks on the CD, reliable retrieval of data requires precision tracking of the serially recorded data. Laser beams are used for both reading as well as tracking the recorded data.
FIG. 1 shows an example of an early two-sided video disc that has video information encoded in a two-state frequency modulated carrier. A 300 mm disc provides 30 minutes of playing time per side. Two aluminum 0.04 .mu.m thick recording surfaces (upper and lower) are bonded together by an adhesive layer and then protected on the upper and lower side by a 1.2 mm transparent base. The binary carrier is recorded as "pits" and "lands" in the reflective aluminum coatings. The track width is 0.4 .mu.m (16 .mu. in) and tracks are separated by 1.6 .mu.m (63 .mu. in). The pits are depressions of 0.1 .mu.m in the reflective coating while the lands are the non-depressed track area between pits.
FIG. 2 shows the main optical assembly including the focus motor and objective lens assembly used to position the objection optics relation to the recording surface. A laser and beam forming assembly illuminates the disc recording surface through the objective lens and the reflected light is deflected by a beam splitter through a cylinder lens to a quadrant photo-detector that is used to sense the state (pit-land) of the recorded track and to adjust the focus by a feedback control signal that adjusts the objective lens position by means the d'Arsonval motor assembly.
This basic method of illuminating, focusing and reading the recorded data is common to all CD-ROMs regardless of the field of application. The actual configuration and data format may vary depending on the application.
Tracking of recorded data tracks on the CD is usually accomplished by one of two methods that use either a single-spot or a multi-spot laser.
The single-spot method of tracking uses one laser beam for both reading and tracking. The lateral position of the beam relative to the track, is modulated (dithered) by a symmetric signal (typically 600 Hz sine wave) producing a small spot deviation (.apprxeq. .+-.0.05 .mu.m) which may be detected by the differences between the two photo-detectors of the quadrant photo-detection of FIG. 2 that are associated with detecting lateral signal contributions. If the laser spot is centered on track, the two lateral photo-detectors should each detect equal affects of the 600 Hz dither. If the laser spot is off-center, one of the two lateral photo-detectors will see a larger affect than the other indicating that the spot is off center-track in the direction associated with the lateral photo-detector receiving the largest 600 Hz component as indicated in FIG. 3.
FIG. 4 shows the single-spot laser method in FIG. 4(a) and the three-spot method in FIG. 4(b). Multi-spot tracking typically uses three laser spots as shown: a center laser spot for reading and a left and right laser spot for detecting the left and right edges of the tracks pits. The center laser spot acts as the source of illumination used to read the recorded data and to provide the necessary focusing information from the four quadrant photo-detector shown in FIG. 2. Tracking spots A and B are placed so that one is centered on one of the lateral edges of the track pits ahead of the spot C and the other is centered on the other lateral edges of the track pits behind spot C. When the CD-ROM is tracking properly, the placement of the spots is as shown in FIG. 4(b). In order to produce tracking spots A and B, two additional laser beams are generated from a common laser source by use of a prism and sent along with the center beam to the recording surface through common optics and the three reflections are diverted by the beam splitter (FIG. 2). Tracking spots (A,B) are each detected by a tracking photo-detector, one mounted to either side of the quadrant photo-detector. The average absolute output of the tracking photo-detectors is equal when spot C is centered on the track and off-balance when not centered. The direction of the off-balance is indicated by the tracking photo-detector sensing the smallest average absolute signal level. The difference of the two tracking sensors is used to control a tri-beam deflection mechanism for centering spot C. In another variation, spots A and B may be used for tracking and focusing, leaving center spot C for data reading only.
The multi-spot tracking method is considered superior to the single-spot method because of the more robust tracking control signal produced which allows the tracking servo-system to respond more rapidly and maintain better tracking control.
The current invention is directed towards improving the multi-spot tracking and reading method by enhancing the data signal by appropriately combining the center and two tracking spot reflected signals without pre-empting the use of the two tracking spot reflected signals normal usage.